Exhaust aftertreatment systems receive and treat exhaust gas generated by an internal combustion engine. Typical exhaust aftertreatment systems include any of various components configured to reduce the level of harmful exhaust emissions present in the exhaust gas. For example, some exhaust aftertreatment systems for diesel powered internal combustion engines include various components, such as a diesel oxidation catalyst (DOC), particulate matter filter or diesel particulate filter (DPF), and a selective catalytic reduction (SCR) catalyst. In some exhaust aftertreatment systems, exhaust gas first passes through the diesel oxidation catalyst, then passes through the diesel particulate filter, and subsequently passes through the SCR catalyst.
Each of the DOC, DPF, and SCR catalyst components is configured to perform a particular exhaust emissions treatment operation on the exhaust gas passing through the components. Generally, the DOC reduces the amount of carbon monoxide and hydrocarbons present in the exhaust gas via oxidation techniques. The DPF filters harmful diesel particulate matter and soot present in the exhaust gas. Finally, the SCR catalyst reduces the amount of nitrogen oxides (NOx) present in the exhaust gas.
The SCR catalyst is configured to reduce NOx into less harmful emissions, such as N2 and H2O, in the presence of ammonia (NH3). Because ammonia is not a natural byproduct of the combustion process, it must be artificially introduced into the exhaust gas prior to the exhaust gas entering the SCR catalyst using a diesel exhaust fluid (DEF) which decomposes into ammonia.
Some prior art exhaust aftertreatment systems, however, do not provide adequate decomposition and mixing of injected DEF. Often, conventional systems cause exhaust gas recirculation within the DEF decomposition tube or low temperature regions within the decomposition tube. Exhaust gas recirculation and low temperature regions may result in inadequate mixing or decomposition, which may lead to the formation of solid DEF deposits on the inner walls of the decomposition tube and DEF injector. Solid DEF deposits include the solid byproducts from incomplete decomposition of urea, such as biuret, cyanuric acid, ammelide, and ammeline. Additionally, inadequate mixing and decomposition may result in a low ammonia vapor uniformity index, which can lead to uneven distribution of the ammonia across the SCR catalyst surface, lower NOx conversion efficiency, and other shortcomings.